Cryogenic vacuum pumps (cryopumps) are widely used in high vacuum applications. Cryopumps are based on the principle of removing gases from a vacuum chamber by having them lose kinetic energy and then binding the gases on cold surfaces inside the pump. Cryocondensation, cryosorption and cryotrapping are the basic mechanisms that can be involved in the operation of a cryopump.
Cryopumps typically use a closed loop helium refrigerator. The refrigerator includes an expander which creates cryogenic refrigeration by the controlled expansion of compressed helium. Cryopumps typically include one or two stages. In a two stage cryopump, refrigeration is produced in a first stage operating at 50K to 80K and a second stage operating at 10K to 20K. Thermally conductive surfaces called cryoarrays are thermally connected to the stages of the expander and are cooled by them.
The refrigerator may be powered by an AC synchronous stepper motor that maintains a constant rotating speed under varying load conditions. This allows refrigeration power to remain stable when line power conditions fluctuate. The refrigerator is designed to provide maximum refrigeration at standard line power conditions.
As noted above, the cryopump can be a one stage or a two stage unit. One stage units are commonly used to condense water vapor and other gases with low vapor pressures at relatively high temperatures. Two stage units are used to remove all gases from a vacuum chamber. These gases are condensed or absorbed on thermally conductive first and second stage arrays attached to the first and second stages, respectively of the expander.
Cryopumps are commonly used in sputtering applications which involve relatively high pressures and a continuous flow of argon. In this application, the temperature of the first stage must be within certain limits for proper operation. The coldest area of the first stage should not reach temperatures below 55K-60K. If the first stage reaches a lower temperature, normally present gases such as nitrogen and argon can temporarily condense on its surface. These gases will slowly migrate to the second stage, causing a phenomenon known as nitrogen or argon "hang up". When the first stage is held at 55K-60K or higher, the first stage does not pump these gases.
In high vacuum applications of cryopumps, pumping of gases occurs primarily during an initial pumpdown period. After reaching the desired pressure, the cryopump essentially idles so as to maintain the desired pressure in the vacuum chamber. During this time, the refrigerator continues to operate at maximum power and may reduce the first stage temperature below the required level. For example, when the gas load is small, the refrigerator may reduce the first stage temperature to less than 40K, whereas 60K is sufficient. Thus refrigeration power is wasted, and "hang up" may occur.
Various approaches have been disclosed in the prior art for controlling the operating temperature of a cryopump. U.S. Pat. No. 4,667,477, issued May 26, 1987 to Matsuda et al, discloses a cryopump wherein the temperature of a cryopanel is regulated by controlling the flow rate of a gas supplied to the expander. In one embodiment, the flow rate is controlled by varying the speed of a compressor which supplies gas to the refrigerator. U.S. Pat. No. 5,001,903, issued Mar. 26, 1991 to Lessard et al, discloses a cryopump wherein the temperature of a first stage is sensed, and the sensed temperature is used to control a heater thermally connected to a second stage. The temperature of a cryopump can also be regulated by controlling the flow rate of gases received from the vacuum chamber as disclosed, for example, in U.S. Pat. No. 4,531,372 issued Jul. 30, 1985 to Slabaugh; U.S. Pat. No. 3,585,807 issued Jun. 22, 1971 to Hengevoss et al; U.S. Pat. No. 4,611,467 issued Sep. 16, 1986 to Peterson; and U.S. Pat. No. 4,285,710 issued Aug. 25, 1981 to Welch. All of the known prior art temperature regulation techniques for cryopumps have had one or more disadvantages, including imprecise temperature regulation, increased power requirements and excessive complexity.
U.S. Pat. No. 4,757,689 issued Jul. 19, 1988 to Bachler et al, discloses a cryopump wherein the pressure within the pump is sensed. When the rate of change of pressure is low, a regeneration process is automatically initiated. Temperature sensors are used to monitor and control the regeneration process.
An Edwards brochure entitled "The Coolstar series . . . variable speed, high capacity cryopumps and microprocessor controlled compressors", 1990, discloses a cryopump having a slow standby speed for extended seal life and a fast boost speed for rapid cooldown and high throughput.